Conventionally known gas separation methods include, for example, (i) chemical absorption, (ii) cryogenic separation, and (iii) adsorption. Although these methods have been widely used, each has merits and demerits.
The chemical absorption method (i) has been used for the removal of hydrogen sulfide or carbon dioxide gas and has also been put to trial use for the desulfurization of exhaust gases. However, this method is defective in that, in the case of using an organic compound as an absorbent, there are problems in treatment of waste fluid, treatment of harmful substances resulting from decomposition of the absorbent, etc. Further, in the case where an acidic gas is treated using a hot aqueous alkali solution as an absorbent, the consumption of heat energy is large.
The cryogenic separation method (ii) has been used, for example, for the separation of air and the separation of hydrocarbon gases such as natural gas. However, this method is disadvantageous in that a large-sized, costly freezing equipment is required. Therefore, practical use of the cryogenic separation method is limited to applications in which separation by the other methods is difficult.
The adsorption method (iii) has been extensively used because it is simple, and the unit used therefor can have a size ranging from small to relatively large. Known types of units for this method include fixed bed type and fluidized bed type.
In adsorption, the amount of a gas adsorbed onto an adsorbent becomes larger with increasing pressure and decreasing temperature, and becomes smaller with reducing pressure and increasing temperature. The adsorption method utilizes this phenomenon in conducting the adsorption step, where a gas is adsorbed onto an adsorbent and the desorption step, where the adsorbed gas is desorbed from the adsorbent. Adsorption separation units of the fixed bed type can utilize the above phenomenon by being provided with a means for changing pressure and temperature. However, in the case of adsorption separation units of the conventional fluidized bed type in which fluidized adsorbent particles circulate in the unit, a pressure difference is rarely utilized in the adsorption-desorption operation. However, a slight pressure is applied as a driving force for circulating the adsorbent particles, and to enable smooth migration of adsorbent particles between the desorption part and the adsorption part. For these reasons, the adsorption-desorption operation in conventional units of the fluidized bed type utilizes a temperature difference only. In the case of adsorption separation units of the fixed bed type, since a larger bed height results in an increased pressure loss, the area of the adsorbent bed should be increased, or the whole unit should be enlarged, in order to heighten treating capacity. However, the possible unit size is limited. Furthermore, size increase of switch valves is also limited.
With a recent increase in the amount of chemical products produced in a single plant in the chemical industry, large amounts of gases need to be treated by gas separation. Therefore, there is a need for an adsorption method capable of coping with such large amounts of gas.
The power consumption in these adsorption processes has been mainly mechanical/electrical type energy. Further, prior art moving bed adsorption processes exhibit an undesirable rate of attrition of the adsorbent particles compared to the fixed and/or stationary bed processes. Additionally, the heat and mass transfer of such processes can be undesirably low. Furthermore, the processes can require an unduly high inventory of expensive adsorbent (particularly as newer sophisticated adsorbents are developed). These and other factors have led to an undesirably high cost of running such prior art processes.